Ping pong ball cannon

What's the best way to build a cannon to shoot an object the size of a ping pong ball when I have these limited materials. I'm really stuck on the best way to make it. Also, it has to have a trigger that is set off by one finger only, and it can't be that you just pull a rubber band back and let go.

Presumably you have to use the rubber bands to provide the energy.
Can you just pull back the bands onto some sort of trigger and then pull the trigger to release them - look up early crossbows in wiki, just about all possible simple trigger latches have been tried.